Hi theimage,
In theory, a pre-pro may have quieter circuitry and more refined features, because the preamp chassis doesn't need to be encumbered by a big power supply and filter section to drive a receiver's many integrated amplifiers.
Wiring layouts can be more generously spaced so there is no residual hum pickup, and preamp-processors generally run very cool compared to a typical A/V receiver that has seven on-board amplifiers and a hefty power supply section.
Cooler running transistors also mean less thermal noise, which will result in impressively low noise floors. There may be more inputs, outputs, and features as well, like a built-in really quiet moving-magnet or moving-coil pre-preamp, for vinyl fanatics.
However, from a practical point of view, I doubt there are any significant audible differences in playback quality with Dolby Digital/dts and multichannel soundtracks between a preamp-processor and a receiver using all the pre-outs when levels between the two are made identical.
